A recording method using an inkjet printer, that is, an inkjet recording method, is a representative method among various color recording methods. An inkjet recording method is to perform recording by generating small ink droplets and attaching the ink droplets to a variety of record-receiving materials (paper, film, cloth, and the like). In this method, since a recording head is not brought into direct contact with a record-receiving material, less noise is generated and silent recording is achieved. Furthermore, since this method has the feature that it is easy to reduce apparatus size and to increase process speed, the inkjet recording method has been rapidly popularized in recent years, and further growth in the future is expected as well.
Conventionally, aqueous inks prepared by dissolving water-soluble dyes in an aqueous medium have been used as inks for fountain pens, felt pens, and the like and as inks for inkjet recording. These aqueous inks generally have water-soluble organic solvents added thereto so that clogging of the ink at pen tips or ink discharge nozzles can be prevented. Further, in regard to these inks, it is required that recorded images with sufficient densities be provided, that clogging at pen tips or nozzles not occur, that the inks have satisfactory dryability on record-receiving materials, that less bleeding occur, that the inks have excellent storage stability, and the like. Furthermore, the water-soluble dyes used therein are required to have high solubility, particularly in water, and to have high solubility in water-soluble organic solvents that are added to the inks. Moreover, the images thus formed are required to have image-fastness properties such as water resistance, light resistance, gas resistance, and moisture resistance.
Among these, gas resistance means resistance to a phenomenon of causing discoloration and fading of a recorded image via an action of ozone gas or the like present in the air and having an oxidizing action on a coloring matter in the record-receiving material. In addition to ozone gas, examples of oxidizing gases having this kind of action include NOx and SOx. However, among these oxidizing gases, ozone gas is regarded as the main causative substance that accelerates the phenomenon of discoloration and fading of inkjet-recorded images, and thus resistance to ozone gas in particular is considered important. At the surface of a paper for inkjet exclusive use capable of giving photographic-image quality, an ink-receiving layer is provided in order to speed up drying of the ink and to reduce bleeding at high image quality. Regarding the material of this ink-receiving layer, materials such as porous white inorganic substances are frequently used. On such a recording paper, discoloration and fading caused by ozone gas or the like is notably observed. Since this phenomenon of discoloration and fading caused by an oxidizing gas is characteristic of inkjet-recorded images, enhancement of gas resistance, particularly ozone-gas resistance, is one of the most important problems to be solved in the field of inkjet recording.
In order to extend the field of application of inkjet recording in the future, there is strong demand for further enhancements of light resistance, gas resistance, moisture resistance, water resistance, and the like in inkjet-recorded images. Furthermore, in addition to this, black images are required to have low color-rendering properties. The phenomenon in which hues seem to change depending on the type of light source is called color-rendering properties, and this phenomenon is likely to occur generally in black dyed materials or record materials. In the field of dye processing, it is common to use compounds having absorption at longer wavelengths in connection with methods for improving the color-rendering properties, and those methods are disclosed in, for example, Patent Documents 7 and 8, and Non-Patent Document 1.
Inks of various hues have been prepared from various coloring matters, but among them black ink is an important ink that is used in both monochromatic images and full-color images. Regarding the coloring matters for such black ink, a large number of coloring matters have been suggested to date; however, it has not been possible to provide coloring matters that adequately fulfill market demand. Many of coloring matters proposed are azo coloring matters, and among them disazo coloring matters such as C.I. Food Black 2 have problems such as poor water resistance or moisture resistance, insufficient light resistance and gas resistance, and high color-rendering properties. Polyazo coloring matters having an extended conjugated system have a problem in that the coloring matters generally have low water-solubility, a bronzing phenomenon in which recorded images partially have metallic gloss is likely to occur, and the coloring matters have insufficient light resistance and gas resistance. In addition, in the case of azo-containing metal coloring matters proposed similarly in large numbers, some have favorable light resistance, but there exist problems of safety for living organisms, unfavorable environmental influences due to metal ions included, extremely inferior ozone gas resistance, and the like.
Examples of black compounds (black coloring matters) for inkjet recording having improved gas resistance, which have been the most important problem to be solved in recent years, include the compounds described in Patent Document 1. These compounds have enhanced gas resistance, but still do not sufficiently fulfill market demand. Furthermore, azo compounds having a benzimidazolopyridone skeleton, which is one of the features of the black coloring matter of the present invention, are disclosed in Patent Documents 2 to 6 and the like. Patent Document 3 discloses trisazo compounds; however, these trisazo compounds have a symmetric structure in which two benzimidazolopyridone skeletons are bonded to the two ends of a linking group containing an azo structure, via another azo structure at each end of the linking group. Thus, compounds similar to the asymmetric azo compound of the present invention are not disclosed therein. Patent Documents 4 and 5 disclose the use of a trisazo compound and a water-soluble black compound for inkjet recording. Furthermore, Patent Document 6 discloses the use of a tetrakisazo compound and a water-soluble black compound for inkjet recording.    Patent Document 1: PCT International Publication No. WO2005/054374    Patent Document 2: PCT International Publication No. WO2004/050768    Patent Document 3: German Patent Application, No. 2004488, Specification    Patent Document 4: PCT International Publication No. WO2007/77931    Patent Document 5: PCT International Publication No. WO2009/69279    Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2008-169374    Patent Document 7: Japanese Unexamined Patent Application, Publication No. H01-284562    Patent Document 8: Japanese Patent Publication No. H05-018955    Patent Document 9: Japanese Patent Publication No. 3383469    Non-Patent Document 1: Processing technology, Vol. 31, No. 9, pp. 599-602, 1996.